A static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This static magnetic field is referred to as the B0 field.
The presence of metallic objects within a subject being imaged can however distort the B0 field. This may cause artifacts within acquired magnetic resonance images.
In Lu et. al., “SEMAC: Slice Encoding for Metal Artifact Correction in MRI,” Magnetic Resonance in Medicine, volume 62, 2009, pp. 66-76, (hereafter “Lu et. al.”) the SEMAC image acquisition technique is explained. The SEMAC technique uses additional phase encoding during the acquisition of the magnetic resonance data to correct for through-plane distortions caused by metallic objects within the subject. The thesis of Chiel den Harder ‘Metal implant artifact reduction in magnetic resonance imaging’ (2014) mentions that at the edge of the acquired volume not all off-resonance image are acquired.